High-accuracy measurement of the fluid level in liquid storage tanks is important for precise inventory control, and also due to growing concern over groundwater contamination that can occur with leakage from such tanks. This is especially true with underground liquid storage tanks (e.g., for gasoline and other petroleum products) because of their proximity to groundwater and the near impossibility of detecting leaks by external inspection. Accurate fluid level measurement, however, is an old problem which has been approached in many ways.
A conventional manual method would involve an operator lowering a weighted tape into the tank until the lower end reached the bottom of the tank so that a reading could be taken. This method is not very accurate or convenient, and can be dangerous if the liquid being measured emits noxious fumes. Simple external transparent tubes or sight gages, from which the liquid level can be visually inspected, can be used on above-ground tanks. Visual inspection, however, is inconvenient, can lead to inaccuracies, and is not well adapted for subsequent automatic processing of data.
The general unsuitability of manual measuring techniques led to the development of automatic liquid-level sensing devices. One category of automatic sensing device employs a float connected by an appropriate series of levers to a dial which provides a liquid-level reading. But floats tend to leak after a period of time, resulting in inaccurate measurements, and the mechanical levers are susceptible to maintenance problems. Sonar or photoelectric systems are another category of automatic sensing device. These systems require expensive and specially designed equipment necessitating maintenance and periodic replacement of parts. Their accuracy is also questionable because the presence of sludge, rust particles or other foreign matter suspended in the liquid will cause inaccurate readings.
Electrical pressure transducer systems are yet another category of automatic sensing device. A very simple transducer system would involve positioning the pressure transducer at the bottom of the tank, and then measuring the pressure exerted on the transducer diaphragm by the column of fluid in the tank above the transducer. This simple system would work well, but for the tendency of transducers to have zero drift (drift in the output signal when no pressure is applied), and sensitivity drift (long-term variations in the output signal when the pressure input is constant). To maintain a high level of accuracy, the submerged transducer must be periodically subjected to zero and sensitivity drift checks.
One approach to providing these periodic checks which has been suggested is depicted in FIG. 2. In this approach, the transducer system is designed to operate in three separate modes. In the measurement mode, one side of the transducer is exposed to atmospheric pressure while the other side is exposed to the pressure developed by the height of the liquid in the tank. In the zero-drift check mode, both sides of the transducer are exposed to atmospheric pressure. In the sensitivity drift-check mode, one side of the transducer is exposed to atmospheric pressure while the other side is exposed to the pressure developed by the height of a column of liquid in a reference standpipe. The reference column of liquid is generated by pumping the standpipe full of liquid from the tank until it overflows and has a height corresponding approximately to the level of liquid in the tank when it is full. While this approach (using a differential pressure transducer in a gage-pressure mode) solves the problem of providing periodic checks on the accuracy of the transducer, it introduces difficulties which could affect the accuracy of the level measurement.
Atmospheric moisture will tend to condense in the vent line which supplies atmospheric pressure to the transducer. A small amount of condensed water in this line will degrade the accuracy of the pressure on the reference side of the transducer during measurement and in the sensitivity and zero-drift check modes. Also, in shifting between the different modes, there will be a tendency for air to get trapped on the liquid side of the transducer and for liquid to get trapped on the air side of the transducer. Either condition will throw off the pressure readings and degrade the accuracy of the system. Finally, a meniscus will tend to form at the upper end of the standpipe. Because the height of the meniscus is not especially repeatable, the accuracy of the system will be adversely affected by changes in the height of the reference column of liquid for successive sensitivity drift checks.
Accordingly, a need exists for an automatic high-precision liquid-level sensing system where periodic checks can be made for zero drift and sensitivity drift to maintain the accuracy of the electrical pressure transducer, and without introducing other factors adversely affecting accuracy of the system.